Large Text Search Help

Hi,
I am trying to design a large text search database.

It will have upwards of 6 million documents, along with meta data on
each.

I am currently looking at tsearch2 to provide fast text searching and
also playing around with different hardware configurations.

1. With tsearch2 I get very good query times up until I insert more
records. For example with 100,000 records tsearch2 returns in around 6
seconds, with 200,000 records tsearch2 returns in just under a minute.
Is this due to the indices fitting entirely in memory with 100,000
records?

2. As well as whole word matching i also need to be able to do
substring matching. Is the FTI module the way to approach this?

3. I have just begun to look into distibuted queries. Is there an
existing solution for distibuting a postgresql database amongst
multiple servers, so each has the same schema but only a subset of the
total data?

Any other helpful comments or sugestions on how to improve query times
using different hardware or software techniques would be appreciated.

Thanks,

Mat